RF phase and attenuator circuits are known where a Field Effect Transistor (FET) is used to process an RF signal. As shown in FIG. 1, generally, an RF shunt circuit 100 includes a FET T1 with a resistor R1 used to couple a control signal Vgate to the gate of the FET T1. In operation, flow of the RF signal from the drain to the source of the FET T1 is controlled by the signal Vgate.
As known, relatively large FETs are used for RF switching in broadband designs in order to provide matching and low insertion loss. These large transistors need about 3 KOhm of resistance on the gate. A FET has a capacitance C that is a function of its size and, as relatively large FETs are used, in order to provide the required RF functionality, the capacitance C is high. The on/off switching time of the FET is a function of the R*C time constant and known FETs switch relatively slowly which affects performance, specifically the time to turn off.
Prior approaches to increasing FET switching speed have included reducing R or C, i.e., the overall time constant, or to change the applied voltages such that the switching time is faster. Reducing the series resistance, i.e., the “R” in the RC time constant, however, results in higher loss in the FET and that loss negatively impacts RF performance. In addition, reducing the series resistance R is not a viable option in phase/attenuator bits because, below a certain value, circuit losses due to the inability to isolate the transistor gate from the RF circuit affect performance.
Further, the size of the switching transistor in phase and attenuator circuits is a fixed value dependent on the specific phase or attenuation that one is trying to achieve. As a result, reducing the transistor size, in order to reduce C, does not work with phase/attenuator bits as the desired phase shift performance might not be achievable.
Another approach places forward and reverse diodes in parallel with the series gate resistor to allow a low-impedance path to the gate during turn-off. This approach requires a path to ground, provided by a large shunt resistor, to limit current flow in the diodes. The parallel diodes approach, however, does not sufficiently isolate the gate for optimal RF signal performance.
What is needed is an approach to speeding up the switching speed of RF FETs without degrading performance.